Perhaps a short explanation of
AMORTIZATION is in order. Forgive me if you already know this.
Although a monthly mortgage payment is a fixed amoaunt, the
actual amounts credited to principal and interest change from month to month. That is, each payment lowers the outstanding balance due (the principal), to which the fixed interest rate is then applied in the following month. In the beginning of the term, very little principal is paid; the payments are mostly interest. As the end approaches, the payments are mostly principal and interest is a small amount. The fixed payment represents the
total of the changing values of these, which does not change monthly.
Take a look here
http://mortgage-x.com/calculators/amortization.htm and you can see how the interest decreases as the loan amount (principal) drops.
Because of the monthly changing of the principal/interest ratio, you can't auto-schedule a fixed amount of either.
Just a guess, but you probably received an AMORTIZATION TABLE when you got your mortgage.